You both lovers decide to marry but the GIRL cant marry due to Family situation!

I have a very close friend.He has a lover and they are in DEEP LOVE since 2 years.They are decided to marry when they are in love and waiting for right time
to inform their parents.
Generally, the GIRL likes her father very much. Now the problem started. The GIRL's father already fixed one boy for her in his relatives and given HIS word to the BOY's family.Here, the GIRL, without knowing herself dragged into the situation where SHE cannot inform her father that she is already in LOVE with some other guy.And, she decided to marry that BOY chosen by her father because she loved her father & mother much where she cannot come out of her family to marry her LOVER.
What to do in this typical situation. Appreciate anyone can give good suggestion whether they should apart or they should follow their family rituals..???
Thanks for all who responds..